This invention relates generally to a fuel cell power supply, and particularly to a fuel cell and a secondary cell connected in parallel to the fuel cell for supplying power to a load while charging and employing the secondary cell as a floating cell. The secondary cell makes up for a deficiency in power supplied by the fuel cell at start up or the peak load. More specifically, this invention relates to a system configuration wherein the output voltage of a fuel cell and the performance of a secondary cell are prevented from decreasing even when an overload condition suddenly intermittently takes place.
When a motor as a load connected to a fuel cell is started, a strong starting load current flows, thus causing sharp fuel consumption across the fuel cell electrodes. Since fuel supply to the electrodes normally takes place, however, the output voltage of the fuel cell is lowered because of the fuel shortage. In the case of hydrogen-oxygen battery system using a fuel quality changer, an unreacted excessive gas or off-gas contained in the fuel hydrogen formed in the quality changer and introduced to the fuel cell electrodes is recycled to the quality changer and used as part of the fuel for an evaporator heating burner. As a consequence, if the consumption of the fuel hydrogen is sharply increased, as mentioned above, the burner will fail to fire because of the off-gas shortage.
In a power supply using fuel cell, a secondary cell is connected in parallel to the fuel cell and used as a floating cell for making up a deficiency in power when the fuel cell fails to bear the full load. In such a power supply including the above described fuel and secondary cells, however, the secondary cell selected is normally equivalent in capacity to the fuel cell, because the former is made to bear an overload over the rated current. For this reason, the secondary cell must be made larger in size.
It is therefore an object of the invention to provide a fuel cell power supply free from reduction in the output voltage of the fuel cell. It is a further object of the invention to prevent fires in an off-gas combustion burner from failing to fire because of an off gas shortage. Another object of the invention is to prevent deterioration in the performance of a secondary cell or damage thereto even when an overload current exceeding the rated output current of the fuel cell is intermittently produced in the load current at short time intervals.
It is another object of the present invention to reduce the capacitance of a secondary cell in a fuel cell power supply, and to make the secondary cell more compact.